The present invention relates to a process for removing nitrogen oxides from gas streams. More particularly, the present invention provides for a process for contacting a flue gas stream containing nitrogen oxides with an ozone-containing oxygen stream in an adsorption/reaction unit, thereby cleaning the flue gas of nitrogen oxides and allowing for recovery of oxygen.
Recent federal and local environmental laws require very significant reduction of discharge of harmful gaseous substances into the atmosphere. Chief among such harmful air pollutants are nitrogen oxides (NOx). In response to strict enforcement efforts of these laws, industrial air polluters have made considerable efforts to reduce the amount of these harmful substances into the air in gaseous effluents from industrial or municipal sources. Successful efforts to reduce the concentration of NOx in gaseous effluents often involve reacting the NOx in waste gases with nitrogen-based reducing agents. One commercially used method of reducing NOx from gas streams involves contacting the NOx with ammonia or an ammonia precursor, such as urea, in the absence of a catalyst, a technique known as selective non-catalytic reduction (SNCR). The ammonia reduces the NOx to nitrogen while itself being oxidized to nitrogen and water. SNCR processes require very high temperatures, for instance temperatures in the range of about 800 to 1200xc2x0 C., and even at these temperatures only low conversions of NOx are achieved. For example, it is not uncommon to attain NOx reductions only in the range of 40 to 50% by SNCR-based processes.
Another technique for removing NOx from waste gas streams involves contacting the waste gas with ammonia or an ammonia precursor in the presence of a substance which catalyzes the reduction of NOx to nitrogen, as in SNCR processes. These catalytic reduction processes are referred to as selective catalytic reduction (SCR). SCR processes have a few advantages over SNCR processes. They can be carried out at temperatures significantly lower than the temperatures at which SNCR processes are carried out. For example, they are quite effective at temperatures in the range of about 250 to 600xc2x0 C. Although SCR processes are more efficient than SNCR processes in the reduction of NOx to nitrogen, they have the disadvantages of being more costly than SNCR processes, the catalyst can be poisoned or deactivated and often they do not remove all of the NOx from the gas stream being treated.
Another disadvantage of both SCR and SNCR processes is that ammonia, which itself is regarded as an environmentally unacceptable pollutant, is often released into the atmosphere in the gaseous effluent from the reactor because the reactions are often conducted in the presence of excess ammonia and/or because of sudden changes in the process that produces less than expected NOx. Ammonia may also be released because of depletion or masking of the catalyst by contamination over time.
Another known method of removing NOx from gas streams involves contacting the NOx with ozone, thereby oxidizing them to higher nitrogen oxides, such as N2O5 and removing the higher oxides from the gas stream by means of aqueous scrubbers.
Specific details of ozone-based NOx oxidation processes are disclosed in U.S. Pat. Nos. 5,206,002 and 5,316,737, the disclosures of which are incorporated herein by reference. Ozone-based NOx oxidation processes can be expensive because of the high cost of producing ozone and require efficient use of ozone to reduce costs.
Because of stringent environmental regulations, efforts are continuously made to improve NOx removal processes to minimize or eliminate emission of NOx into the atmosphere. This invention provides a process which accomplishes this objective.
The present invention provides for a process for removing nitrogen oxides (NOx) from gas streams, particularly flue gas streams. The process comprises feeding a gas stream that contains nitrogen oxides as an impurity therein to an adsorber where the nitrogen oxides are adsorbed during the adsorption cycle. During the regeneration cycle an ozone/oxygen stream is fed to the adsorber. The ozone will react with the adsorbed nitrogen oxides and form N2O5. The N2O5 will flow out from the adsorber with the oxygen stream to a scrubber system such that the oxygen can be recycled as feed to the ozone generation unit. The treated flue gas stream can be discharged to the atmosphere or directed to other treatment systems.
In another embodiment of the present invention, the nitrogen oxides-containing stream may be a chemical processing stream such as a metal pickling, FCC or food processing stream.